Lithographic printing plates (after process) generally consist of ink-receptive areas (image areas) and ink-repelling areas (non-image areas). During printing operation, an ink is preferentially received in the image areas, not in the non-image areas, and then transferred to the surface of a material upon which the image is to be produced. Commonly the ink is transferred to an intermediate material called printing blanket, which in turn transfers the ink to the surface of the material upon which the image is to be produced.
At the present time, lithographic printing plates (processed) are generally prepared from lithographic printing plate precursors (also commonly called lithographic printing plates) comprising a substrate and a photosensitive coating deposited on the substrate, the substrate and the photosensitive coating having opposite surface properties. The photosensitive coating is usually a photosensitive material, which solubilizes or hardens upon exposure to an actinic radiation. In positive-working systems, the exposed areas become more soluble and can be developed to reveal the underneath substrate. In negative-working systems, the exposed areas become hardened and the non-exposed areas can be developed to reveal the underneath substrate. Conventionally, the actinic radiation is from a lamp (usually an ultraviolet lamp) and the image pattern is generally determined by a photomask that is placed between the light source and the plate.
Laser sources have been increasingly used to imagewise expose a lithographic printing plate that is sensitized to a corresponding laser. This allows the elimination of the photomask film, reducing material, equipment and labor cost. Among the lasers useful are infrared lasers (about 830 nm or 1064 nm), FD-YAG laser (about 532 nm), violet laser (about 405 nm), and ultraviolet laser (such as about 375 nm). Violet and ultraviolet laser sensitive plates are quite attractive because of their yellow light handling capability (in contrast to red light handling for FD-YAG sensitive plate) and higher quantum efficiency (than infrared laser sensitive plate). However, the energy output of a violet or ultraviolet laser diode is much lower than that of an infrared laser diode. In order to be imaged with a violet or ultraviolet laser imager at a practical speed, a violet or ultraviolet laser sensitive plate should have a sensitivity of preferably less than 300 μJ/cm2, more preferably less 200 μJ/cm2, and most preferably less than 100 μJ/cm2, which is significantly lower than the sensitivity required for an infrared laser sensitive plate (typically 50 to 300 μJ/cm2) or a conventional plate exposed with an ultraviolet lamp through a photomask (typically 50 to 300 mJ/cm2).
Silver halide based violet laser sensitive plate has recently been introduced as the first lithographic printing plate suitable for imaging with violet laser, utilizing the inherently high sensitivity of silver halide. Examples include U.S. Pat. No. 6,541,176. However silver halide based plates have the disadvantages of relatively low run length and generating hazardous silver waste.
Photopolymerizable composition based (also called photopolymer) violet or ultraviolet laser sensitive plates are very attractive because of the high durability of photopolymer plates. However, because of the extremely high sensitivity requirement of the violet or ultraviolet laser sensitive plate, a commercially viable violet or ultraviolet laser sensitive photopolymer plate can not be achieved by simple addition of a laser absorbing sensitizer into a conventional photopolymer plate but will require certain specific photopolymerizable composition. The selection of the monomer, polymer, and intiator is crucial in achieving a commercially viable violet or ultraviolet laser sensitive photopolymer plate.
U.S. Pat. No. 6,689,537 (Urano et al) describes some specific compositions for violet photopolymer plate, wherein the photosensitive layer comprises multifunctional (with 3 or more functional groups) urethane acrylate monomers, difunctional non-urethane acrylate monomers, a polymeric binder and a hexaarylbiimidazole compound, and the weight ratio of the multifunctional monomers to the polymeric binders is less than 0.75.
U.S. Pat. No. 6,423,471 (Sorori et al) describes some specific compositions for violet photopolymer plate, wherein the photosensitive layer comprises a monomer and a polymeric binder at a weight ratio of 0.75, with a titanocene compound as the free radical initiator.
U.S. Pat. No. 6,841,336 (Kondo) describes some specific compositions for violet photopolymer plate, wherein the photosensitive layer comprises a multifunctional monomer and a polymeric binder at a weight ratio of 0.75, with a titanocene compound as the free radical initiator.
U.S. Pat. App. No. 2006/0046191 (Sataka) describes some specific compositions for violet photopolymer plate, wherein the photosensitive layer comprises a multifunctional monomer and a polymeric binder at a weight ratio of 1.0, with a titanocene compound as the free radical initiator.
U.S. Pat. No. 6,749,995 (Matsumura) describes some specific compositions for violet photopolymer plate, wherein the photosensitive layer comprises polyfunctional urethane acrylate monomers, trifunctionality non-urethane acrylate monomer and polymeric binder; the weight ratio of the multifunctional monomers to the polymeric binders is 1.0, and the weight ratio of the polyfunctional urethane acrylate monomer to the polymeric binder is 0.25. A specific phosphorus-containing compound, a specific nitrogen-containing compound, or a titanocene compound is used as the free radical initiator.
U.S. Pat. No. 7,011,928 (Takamuki) describes some specific compositions for violet photopolymer plate, wherein the photosensitive layer comprises a monofunctional acrylate monomer, a difunctional methacrylate monomer, a multifunctional urethane (meth)acrylate monomer, and a polymeric binder, with a borate salt as the free radical initiator; the weight ratio of the multifunctional urethane monomer to the polymeric binder is 0.625.
U.S. Pat. No. 7,041,429 (Ohta, et al) describes some specific compositions for violet photopolymer plate, wherein the photosensitive layer comprises a difunctional methacrylate monomer, a multifunctional urethane methacrylate monomer, and a polymeric binder; the weight ratio of the monomers to the polymeric binder is 1.0 and the weight ratio of the multifunctional (meth)acrylate monomer to the polymeric binder is 0.625. A trihalomethyl group-containing oxadiazole compound, with or without addition of a titanocene, monoalkyltriaryl borate, or iron arene complex compound, is used as the free radical initiator.
U.S. Pat. App. Pub. No. 2003/0186165 (Gries et al) describes some specific compositions for violet photopolymer plate, wherein the photosensitive layer comprises difunctional (meth)acrylate monomer and a polymeric binder at a weight ratio of about 1.6 to 2.4, with a hexaarylbiimidzole compound as the free radical initiator; while photosensitive layers having a trifunctional non-urethane monomer and a polymeric binder at a weight ratio of about 1.63 or having a penta functional non-urethane acrylate monomer and a polymeric binder at a weight ratio of about 1.50 are also described in the patent application as comparative examples, such photosensitive layers were reported in the patent application to give no hardening upon exposure to the violet laser.
U.S. Pat. App. Pub. No. 2006/0024614 (Williamson) describes some specific compositions for violet photopolymer plate, wherein the photosensitive layer comprises a difunctional (meth)acrylate monomer and a polymeric binder, with a hexaarylbiimidzole compound as the free radical initiator; no specific photosensitive composition containing multifunctional (meth)acrylate monomer was disclosed.
While multifunctional monomers and higher content of the total monomers in an aqueous alkaline developable violet plate are described in the patents, there is no teaching of higher weight ratio of the multifunctional monomers to the polymeric binders for such plate. Apparently, higher content of the multifunctional monomer might be thought to cause difficulty in development with aqueous alkaline developer, since multifunctional monomers are generally larger in size and less soluble or dispersible in aqueous developer than monofunctional or difunctional monomers.
The inventor has found, surprisingly, violet or ultraviolet laser sensitive lithographic printing plates having, between a hydrophilic substrate and a water soluble or dispersible overcoat, a photosensitive layer comprising multifunctional (meth)acrylate monomer and polymeric binder at higher weight ratio can be developed with an aqueous alkaline developer and give significantly better photospeed and press durability than plate with lower monomer-to-polymer weight ratio or with lower monomer functionality. In addition, the process conditions for developing the plate and for preparing the photosensitive coating fluid have also been found to be important in achieving good performance for the specific violet or ultraviolet laser sensitive photopolymer plates.